1. Field of the Invention
The present invention relates to a time division communication system of radio communication channels in a mobile communication system. More particularly, the present invention relates to a time division communication system of radio communication channels in a mobile communication system in which one of many radio mobile stations within a service area, to which radio channels are allocated, set up wireless circuits connecting to radio base stations by using the allocated radio channels, and when the first mobile stations are communicating with the base stations through the wireless circuits, another mobile station requests a communication with the base station currently engaging the communication with the first mobile station, by using the same radio channel as the first radio channel, a wireless circuit can be set up between the second mobile station and the base station in a transmitting/receiving diversity communication mode by the same radio channel or another radio channel, without any adverse effect on the now progressing communication between the first mobile station and the base stations.
2. Description of Prior Art
A conventional mobile communication system has been employed in a land mobile telephone system commercially serviced by NTT (Nihon Telephone and Telegram Co., Ltd). The telephone system will be described with reference to FIG. 16. A plurality of radio channels are allocated to a radio base station 13, in order that it communicates with a number of mobile stations 15 carried on vehicles, which roam in a zone 14 as a service area. Each mobile station 15 has a function to select one of the radio channels (this function is called a multi-access). When the mobile station 15 desires to communicate with the base station 13, the station 15 sends a control signal to a wireless circuit control station 12 by way of the base station 13. The control station 12 determines radio channels used by many base stations 13. In response to an instruction from the control station, the mobile station 15 determines a speech channel number to be used for communication, and communicates with a subscriber in a public telephone network 10, through an exchange 11 including switches SW.
In the field of wireless communication, a transmitting/receiving diversity technique is frequently used. Many diversity techniques have been known. In a frequency diversity, at a transmitting point, a plurality of transmitters simultaneously transmit the same signal at different frequencies. At a remote receiving point, a plurality of receivers, tuned to the transmitting frequencies, receives the transmitted signals, detect them, and adds together the detected signals. In a transmission space diversity, at a transmitting point, an output signal of a transmitter is divided coupled with a plurality of antennae disposed at different locations. These divided signals are transmitted from the antennae. At a distant remote receiving point, the transmitted signals are received by a single antenna, led to a receiver, and detected in the receiver. In a receiving diversity, at a transmitting point, an output signal of a transmitter is led to a single antenna, and is transmitted by the antenna. At a remote receiving point, the signal is received by a plurality of antennae disposed at different locations, and led to a receiver. After the received signals are passed through a high (intermediate) frequency stage or a detect stage, the received signals are added together.
In the transmitting/receiving diversity, amplitude modulation or angular modulation is used for modulating a signal.
A communication system of the type in which the base station and the mobile station use the same transmitting frequency, that is, the same radio frequency is used as a transmitting/receiving frequency, is also used in the digital mobile communication system.
In the communication system, if the number of radio channels for speech as assigned to a base station is 10, those radio channels can be allocated to communication requests by 10 number of mobile stations within a service area. The communications of the base station and the mobile stations can be done, while being free from radio interference. For an 11th call originating request generated by a mobile station, the base station cannot originate a call (call loss) because there is no radio channel assigned to the mobile station. The above description of problem relates to the case where the radio channel is used for transmitting an analog signal. The same problem is involved in the communication system using a voice signal subjected to digital modulation process, and the system of the single channel per carrier (SCPC) type in which a telephone (communication) signal is transferred by a single carrier wave.
Further, in the system using only assigned radio channels, the band width of the signal is fixed. Accordingly, it is impossible to transmit a signal whose band width is broader than that of the assigned channel.